The Thermoelectric Cooler, or TEC, is well known as a type of electronic heat pump to those skilled in the art of heat transfer. A major difficulty in using these in refrigeration devices is that as the temperature differential across the TEC becomes greater, its efficiency decreases. Adding TEC units, to make up for the loss of efficiency, simply drives efficiency lower, to the point that cost, packaging considerations and operating expense render TECs impractical for all but very small refrigeration units.
Another problem with these devices is that thermal stresses often cause cracking. This can come about because of expansion and contraction across the hot and cold plates to which the devices are mounted, wherein abrupt changes of drive current leads to thermal shock. The amount of work done by each junction of a TEC s a function of the temperature across it. Inasmuch as all junctions in a single TEC are in series connection, and in many cases multiple TECs are also in series connection, the overall efficiency is the product of the junction efficiencies and overloading of one or several junctions will lead to thermal stress and related problems.
In removing heat from the hot side, or conducting heat to the cold side, an increase in the physical size of the plate and an increase in the volume and/or velocity of air flowing over this plate will result in an increase in the heat energy transferred. This reduces the temperature difference across the TEC.
A problem in achieving this solution is found in the thermal resistance of any thermally conductive material. Any increase in the distance traveled or the amount of heat energy transferred will cause a greater temperature differential and a decrease of overall efficiency. The flow of heat through a material creates a temperature drop that adds to the TEC temperature differential and results in a decrease of system efficiency.
Heat pipes are also well known to those skilled in the art. Whatever heat energy is absorbed in the process of evaporation of a fluid contained in a heat pipe is released when the fluid condenses. The only thermal resistance will be that of the tube walls of the device. Heat can be absorbed at a high transfer rate at a concentrated area and released over a greater area at a lower rate of transfer per unit area as long as the amount of heat “in” equals the amount of heat “out”. This will work in both directions, so that this effect can be used to move heat to or from a TEC with a minimum temperature differential due to thermal resistance. Any point above the average temperature of the total heat pipe will act as an evaporator, and any point below the average temperature of the heat pipe will act as a condenser. With the resulting transfer of heat energy, the temperature of all points on the heat pipe will become equal, neglecting the effects of any thermal resistance due to its construction.
The amount of heat that can be transferred by a heat pipe is a function of the latent heat of vaporization of the fluid used and the volume of working fluid that can physically be evaporated, transported, and condensed per unit time. Fluid volume processing capacity is limited by the heat transferring areas of the evaporator and condenser and the physical capacity of the heat pipe to transport vapor and fluid.
Prior art devices have addressed the problem of heat transfer to and from the TEC by respective heat pipes by using common working fluid evaporator or condenser volumes to interface with a grouping of TECs. The inherently unequal distribution and inefficient fluid flow characteristics cause unequal TEC load distribution as a basic problem in such a configuration. In addition, since heat pipes commercially available only as closed end tubes, manufacturing costs of such a configuration are excessive for commercial applications. This is especially true if the heat pipes are of the wicked and cored type, as are desirable for this application. Osmotic or mechanically pumped heat pipes introduce added complexity and expense to a device. Loop configuration heat pipes will have thermal gradients from top to bottom, inasmuch as this is the mechanism used to cause the fluid to rise in one arm of the loop and fall in the other. In this application, thermal gradients may cause thermal stress and unequal sharing of heat pumping loads in the TECs as described above. Basic open thermo-syphon configurations, without core or wicking, are low efficiency devices because of liquid pooling and thermal resistance effects in the fluid itself. Another problem is that as the fluid evaporates, it forms bubbles on the walls of the evaporator section that insulate the wall from the fluid. At the condensing end of a thermo-syphon, as the fluid becomes a liquid, the droplets interfere with contact of the vapor to the wall, again reducing efficiency. Any increase of the amount of heat energy to be transferred increases the magnitude of the problems in a thermo-syphon.
TECs are electronic heat pumps. One of the characteristics that often causes difficulties in employing these for refrigeration is that the amount of heat pumped per Watt of electricity used has an inverse relationship to temperature across the device. For example a particular TEC may pump 204.8 BTU/H at a temperature of 10C and an electrical input of 95.8 Watts, but if the temperature becomes 30C, this same device may only pump 136 BTU/H with the same 95.8 Watts of electrical input.
The relationship between amount of heat pumped (BTU/H) and the temperature differences developed across any material this heat flows through is “thermal resistance”—RTH. If T=temperature, RTH=thermal resistance, and heat flow through the material is in BTU/H the relationship is: T=BTU/H×RTH. This points to the main reason that thermoelectric devices are not more commonly used for refrigeration. As we attempt to move heat, the effect of the thermal resistance will cause a rise in temperature across all elements of the apparatus through which the heat must flow and this will degrade the ability of the TEC to pump this heat.
Although metals in general are good heat conductors, the use of aluminum, or even copper, as heat sinks will build the thermal resistance of these metals into a refrigeration system. This results in a significant temperature differential between the TEC and the radiator or absorber over the distances involved in coupling the small TEC to the large surface area needed to collect or dissipate heat. In the past this has generally limited TEC systems to the cooling of small enclosures or maintaining a small device at a regulated low temperature,
A first object of the invention therefore, is to provide a TEC driven refrigeration system with sufficient capacity for use in typical home and commercial applications. A second object is that the operating costs of this TEC driven refrigeration system be equal or less than prior art mechanical systems. A third object is that this TEC driven refrigeration system be no more expensive to manufacture than prior art mechanical systems and yet another object is to mitigate the problems associated with thermal stresses so that these systems be more reliable in operation than prior art systems.